Electric motors (hereinafter simply referred to as “motors”) such as brushless DC motors and AC synchronous motors are typically driven by three phase currents. A complicated control technique, such as vector control, is needed to accurately control the waveforms of the three phase currents. Such a control technique requires complicated mathematical calculation and is therefore performed using a digital computation circuit, such as a microcontroller (microcomputer). The vector control technique is utilized in the fields of applications in which the load on a motor varies significantly, such as washing machines, motorized bicycles, electric scooters, electric power steering devices, electric cars, and industrial equipment. Meanwhile, other motor control techniques, such as pulse width modulation (PWM), are employed for motors that have a relatively low output.
In the field of vehicle-mounted devices, an automotive electronic control unit (ECU) is used in a vehicle. The ECU includes a microcontroller, a power supply, an input/output circuit, an A/D converter, a load drive circuit, and a read only memory (ROM), etc. An electronic control system is constructed using the ECU as a main component. For example, the ECU processes a signal from a sensor to control an actuator, such as a motor. More specifically, the ECU controls an inverter in a power conversion device while monitoring the rotational speed or torque of a motor. The power conversion device converts drive power that is to be supplied to the motor, under the control of the ECU.
A mechanically and electronically integrated motor in which a motor, a power conversion device, and an ECU are integrated together has in recent years been developed. In particular, in the field of vehicle-mounted devices, high quality needs to be ensured for safety. Therefore, a fault-tolerant design is employed in order to allow the motor system to continue a safe operation even if some part of the motor system fails. As an example of such a fault-tolerant design, a single motor may be provided with two power conversion devices. As another example, the ECU may be provided with a backup microcontroller in addition to a main microcontroller.
For example, Patent Document No. 1 describes a power conversion device for converting power that is to be supplied to a three-phase motor, the device including a control unit and two inverters. The two inverters are each coupled to a power supply and a ground (hereinafter referred to as a “GND”). One of the two inverters is coupled to one end of each of the three phase windings of the motor, and the other inverter is coupled to the other end of each of the three phase windings. Each inverter includes a bridge circuit that includes three legs each including a high-side switching element and a low-side switching element. The control unit, when detecting a failure in a switching element in the two inverters, switches the control of the motor from control under normal conditions to control under abnormal conditions. As used herein, the term “abnormal conditions” mainly means that a switching element has failed. The term “control under normal conditions” means control that is performed when all the switching elements are operating normally. The term “control under abnormal conditions” means control that is performed in the event of a failure in a switching element.
In the control under abnormal conditions, a neutral point for the windings is formed by turning on and off switching elements according to a predetermined rule in one of the two inverters that includes a switching element that has failed (hereinafter referred to as a “failed inverter”). According to the rule, for example, in the event of an open-circuit failure in which a high-side switching element is always off, the three high-side switching elements other than the failed switching element are turned off, and the three low-side switching elements are turned on, in the bridge circuit of the failed inverter. In this case, the neutral point is formed on the low side. In the event of a short-circuit failure in which a high-side switching element is always on, the three high-side switching elements other than the failed switching element are turned on, and the three low-side switching elements are turned off, in the bridge circuit of the failed inverter. In this case, the neutral point is formed on the high side. In the power conversion device of Patent Document No. 1, the neutral point for the three phase windings is formed in a failed inverter under abnormal conditions. Even in the event of a failure in a switching element, the motor can continue to be driven using one of the inverters that is operating normally.